Chlorobenzene hydrodechlorination on bimetallic catalysts prepared by laser electrodispersion of NiPd alloy

Abstract

NiPd bimetallic systems were for the first time synthesized by laser electrodispersion (LED) of the Ni77Pd23 alloy target followed by the deposition of produced bimetallic particles on a TEM copper grid and alumina granules. Selective area energy-dispersive analysis confirms the bimetallic nature of NiPd particles deposited on a TEM copper grid. Their mean size is 1.0 nm according to TEM. XPS data demonstrate that under deposition on alumina granules (total metal content of 0.005 wt.%), nickel in bimetallic particles nearly completely oxidizes to Ni2+ species predominantly in the form of aluminate. At the same time major part of palladium (84%) exists in Pd0 but oxidizes to Pd2+ (80%) during 6 months storage in air. Both metals are deposited on the external surface of alumina granules and localized in the same areas. In situ reduction of both metals by H2 in the catalytic cell of XPS spectrometer is hindered. Nickel is not reduced even at 450°C, confirming the formation of NiAlOx, whereas palladium is reduced at higher temperatures compared to a similar monometallic catalyst. Nevertheless, NiPd/Al2O3 catalyst is more efficient in gas-phase chlorobenzene hydrodechlorination at 150–350°C than Ni/Al2O3 and even Pd/Al2O3, and much more stable. The difference may be caused by the formation of new active sites due to the contact between Pd0 and NiAlOx-modified support, and the protective action of spinel reacting with HCl by-product.

Bimetallic Nanostructured Catalysts Prepared by Laser Electrodispersion: Structure and Activity in Redox Reactions

“…The uniform distribution of size-selected NiPd particles of about 1 nm in size was found in bimetallic catalysts deposited by LED from the Ni 77 Pd 23 alloy. [14] According to STEM/EDS data for NiPd/Al 2 O 3 ( Figure S2, SM), both metals were located at the same areas on the catalyst surface and in all analysed areas nickel prevails over palladium about 3 times. These data may be considered as an evidence for the formation of bimetallic particles, or at least both metals are situated in close vicinity.…”

“…[24] But peaks of oxidized states were observed at binding energies at 856.0 eV for Ni2p and 338.0 eV for Pd3d that are higher than the values of 854.5 eV for NiO [25] and 336.7 eV for PdO. [24] Such shift can be explained by metalsupport interaction (MSI) which may even lead to the formation of nickel aluminate [14] or other mixed oxides [26] It is reasonable to assume that the MSI may be diminished by replacing alumina with a more inert carbon support. To check this assumption two carbon materials -HOPG and Sibunit were used for metal deposition in this work.…”

“…Recently the possibility to produce a bimetallic coating on the alumina surface using the NiPd alloy target for LED deposition has been shown. [14] These bimetallic catalysts with the total metal loading of 0.005 wt.% demonstrated higher activity and stability in chlorobenzene hydrodechlorination than their monometallic Pd counterparts. Monometallic Ni LED catalysts showed low activity and were rapidly chlorinated in this reaction.…”

“…However bimetallic LED catalysts were synthesized only recently. [14,15] The PdÀ Ni active component is one of the most interesting for catalytic systems. Recently the possibility to produce a bimetallic coating on the alumina surface using the NiPd alloy target for LED deposition has been shown.…”

“…Monometallic LED catalysts comprising Pd, Pt, Ni, Cu supported on carbon and oxide supports were widely studied in several classes of catalytic reactions, and very promising results were achieved in environmentally important reactions of hydrodechlorination of chlorinated organic molecules. However bimetallic LED catalysts were synthesized only recently …”